1. Field of the Invention
This invention relates to delay lines in optical coherence tomographic and optical Doppler tomographic systems, and dynamic focusing mechanisms in confocal microscopy and optical coherence microscopy.
2. Prior Art Statement
Confocal microscopy, optical coherence tomography (OCT), and optical coherence microscopy (OCM) are novel optical tomography techniques which are very useful for providing subsurface high-resolution imaging of samples (including but not limited to biological and medical samples). Confocal microscopy can achieve a sub-micron resolution and a penetration depth up to a few hundred microns. Optical coherence tomography can provide a spatial resolution up to a few microns and a penetration depth up to a few millimeters. Its advantages over confocal microscopy are the higher sensitivity due to signal enhancement by optical interferences, and a faster image acquisition rate because the axial scanning is obtained by an optical delay line in the reference arm, which is generally faster than traditional mechanical scanning stages. Optical coherence microscopy is a combination of confocal microscopy and optical coherence tomography. It uses a high numerical aperture lens to reduce the spot size of the focal point in order to obtain a better spatial resolution than OCT, and low coherence interferometry to reject multiple scattering lights. However, the axial scanning range is limited by a much shorter Rayleigh range. As a consequence, an additional translation stage is always needed to achieve adequate axial scanning range.
In many potential biomedical applications, the data acquisition speed is a critical issue in suppressing motion artifacts and acquiring high resolution four dimensional images (three spatial dimensions and one temporal dimension). Rapid delay lines are necessary to achieve fast OCT, while dynamic focusing mechanism instead of mechanical scanning stages is desirable for fast confocal microscopy and OCM.
A primitive delay line is a translating mirror, which is driven by a linear motor, an actuator, or a piezoelectric transducer (PZT). As the mirror moves back and forth along the path of the received optical signal, the power consumption required to generate acceleration will increase dramatically with frequency and scanning range. This is the reason that most commercially available linear motors and actuators can only provide a repetition rate around 30 Hz when a 2-3 mm scanning range is required. Although PZT can be driven at much higher frequencies, they can provide a limited scanning range. Resonant scanners have been demonstrated to achieve a frequency of 1,200 Hz and up to a 3 mm optical length difference. The drawback is that the optical path length change is a time-dependent sinusoidal function. As a result, Doppler frequencies of interference signals are depth dependent and vary within a wide range, which may cause difficulties in signal filtering and introducing more noises.
Sophisticated delay lines require complicated arrangements of mirrors, gratings and/or lenses, as well as precise alignment. Grating based delay lines have the flexibility to adjust group delay and phase delay independently. Repetition rates of 2,000 scans/second and 4,000 scans/second have been reported for such delay lines with a galvanometer (driven with a 1-kHz triangle waveform) and a 4-kHz resonant scanner, respectively. It appears that without using resonant scanners, the vibrational motion based mechanical scanning cannot readily achieve a speed high enough to meet real time data acquisition requirements. Rotating cubes, rotating roof prisms, and a combination of a polygonal mirror and a glass cube can scan up to 28.5 kHz. However, these methods suffer from rather low duty cycles and/or considerable nonlinearity of optical path length change.
Recently, an OCT system without any moving parts for depth scanning was proposed, and a high repetition scanning rate of 500 kHz was achieved in a scanning range of 25 mm by using optical frequency comb generators. However, the depth resolution (100 microns) and signal to noise ratio of this system needs to be improved. In addition, the cost of this system is high due the use of expensive components, such as gigahertz electronics and electro-optical modulators. Some fast delay lines are linear and can achieve several kHz scanning speed. However, they suffer from wavelength dependent group velocity dispersion.
A fast scanning device is necessary for high-speed microscopic imaging methods such as optical coherence tomography (OCT) and confocal microscopy. In an OCT system, axial scanning is generally achieved with a variable optical delay line, whose repetition rate determines the image acquisition speed. In a confocal microscope, angular scanning of collimated beam is transformed into lateral scanning of focus inside a sample. Conventional scanners cannot readily achieve kilohertz repetition speed at a reasonable cost and acceptable performances.
A widely used delay line for OCT is based on a grating and a scanning mirror that has a varying tilting angle, as disclosed in U.S. Pat. No. 6,111,645A (Tearney et al.). The reported axial scanning rate was 2 kHz. The use of a grating is critical for converting angular beam scanning into optical path length change. However, dispersion of the grating may degenerate the resolution of the system and cause problems when multiple wavelengths are needed for spectroscopic information. In addition, non-linearity in scanning speed is inevitable when resonant scanners are used for kilohertz repetition rates.
A 2.58 kHz reflectometer comprised of a rotating polygon mirror was disclosed in an article entitled xe2x80x9cRobust and rapid optical low-coherence reflectometer using a polygon mirrorxe2x80x9d by Delachenal et al. (Optics Communications, 162 (1999) pp. 195-199). The high scanning speed comes at the cost of poor linearity and a low duty factor. The same problems are related to the optical delay line with a rotating cube that was disclosed in an article entitled xe2x80x9cAchieving variation of the optical path length by a few millimeters at millisecond rates for imaging of turbid media and optical interferometry: a new techniquexe2x80x9d by Su (Optics Letters.22, (1997), pp. 665-667).
Recently, an OCT system without any moving parts for depth scanning was disclosed in an article entitled xe2x80x9cUltrahigh scanning speed optical coherence tomography using optical frequency comb generatorsxe2x80x9d by Lee et al. (Japanese J. of Applied Physics, Part 2, 8B, (2001), L878-880). A fairly high repetition scanning rate of 500 kHz was achieved in a scanning range of 25 mm by using optical frequency comb generators. However, the depth resolution (100 microns) and signal to noise ratio of this system cannot meet requirements for biomedical applications. In addition, the cost of this system is high due to the use of expensive components, such as gigahertz electronics and electro-optical modulators.
One example of linear scanning optical delay line was disclosed in U.S. Pat. Nos. 5,784,186A and 5,907,423A (Wang et al.). A helicoid reflecting mirror was used as a linear scanning line in an optical second-harmonic generation autocorrelator. The scanning speed was reported as 43.5 Hz. Fabrication of the spiral reflecting surface would be expensive when a high accuracy and high reflectivity are required.
Another example relates to two oppositely lying reflection means that was disclosed in U.S. Pat. No. 6,341,870B1 (Koch et al.). A movement of one mirror with respect to another of 45 microns is enough to results in a path length change of 2 mm. However, the overall path length and path length change are very sensitive to orientation of the incident beam with respect to the mirrors. Very accurate alignment and vibration control may be required.
A further example of another design relates to an optical path length scanner using moving prisms that was disclosed in U.S. Pat. No. 6,407,872B1 (Lai et al.). The design was tested with Zemax simulation but no experimental validation has been reported. It also has the dispersion problem associated with the use of prisms. In addition, such a device cannot be used as a scanning device in confocal microscopy.
A number features, which are necessary for enhanced utilization of these techniques commercially in medical and other applications include the following; ability to linearly change the optical path length or the linear axial scanning of the focus inside a sample in the several millimeter range; high scanning speeds without sacrificing quality; dispersion free operation; easy to align; high duty factors; robust to provide long lifetimes; and have a structure that is easy to fabricate and inexpensive compared to alternatives. The aims of the present invention are to provide these features.
It is an objective of the present invention to provide a device which at high speed can linearly change an optical path length up to several millimeters.
It is another objective of the present invention to provide a device which at a high scanning speed can do linear axial scanning of a focus inside a sample over a range of several millimeters.
It is still another objective of the present invention to provide a high speed, axially, linear scanning device which is dispersion free, easy to align, can be used at a high duty factor, easy to fabricate, low cost as well as being robust and having a long lifetime.
In one embodiment, an optical coherence tomography system comprises a radiation source generating a beam of radiation; a waveguide system, or a beam splitter, receptive of the beam of radiation which splits the beam of radiation into a sample beam and a reference beam and recombines the sample beam as a return sample beam and the reference beam as a return reference beam into a composite beam. A delay mechanism is receptive of the reference beam and introduces a relative time delay between the sample beam and the reference beam.
In another embodiment, an optical coherence microscopy system comprises a radiation source generating a beam of radiation. A waveguide system, or a beam splitter, is receptive of the beam of radiation and splits the beam of radiation into a reference beam and a sample beam and recombines the sample beam as a return sample beam and the reference beam as a return reference beam into a composite beam. The composite beam is indicative of the interference between the reference beam and the sample beam. A phase modulator is receptive of the reference beam for generating fringe signals and a dynamic focusing mechanism is receptive of the sample beam for scanning the focal point inside the sample. The reference beam may be blocked to reduce the optical coherence microscopy system to a confocal microscopy system.
An optical scanning mechanism, for optical delay or dynamic focusing is described. The optical scanning mechanism comprises a set of reflectors receptive of a signal wherein the reflectors are positioned at a prescribed angle with respect to a plane of motion and a device for causing linear or rotational relative motion between the set of reflectors and the signal toward the plane of motion.
Briefly stated, the present invention provides a high speed, high duty cycle, linear, optical scanning device suitable for optical coherence tomography, optical coherence microscopy and confocal microscopy is presented. For the microscopy applications stable, periodic scanning is achieved by using a rotary mirror array, having a rotational symmetry and mirrors tilted at a small angle with respect to the rotational plane. The rotary mirror array is rotated at a constant speed. For the tomography application periodic modulation of the optical path-length of the reference beam is controlled by the rotation of the rotary mirror array.
The above and other objects, features, and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings. In which like reference numbers in different drawings denote like items.